A heat pump device is known in the art which uses the property of an elastic member of rubber or any other material that generates heat when allowed to expand adiabatically and that absorbs heat when allowed to contract adiabatically (see, for example, Patent Documents 1 and 2). If such a heat pump device is applied to an air conditioner, the air conditioner is allowed to perform a heating mode of operation by supplying a room with the air heated during the adiabatic expansion of the elastic member, and a cooling mode of operation by supplying the room with the air cooled during the adiabatic contraction of the elastic member.